In many people who are profoundly deaf, the reason for deafness is absence of, or destruction of, the hair cells in the cochlea which transduce acoustic signals into nerve impulses. These people are thus unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to, or an absence of the mechanism for nerve impulses to be generated from sound in the normal manner.
It is for this purpose that cochlear implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. U.S. Pat. No. 4,532,930, the contents of which are incorporated herein by reference, provides a description of one type of traditional cochlear implant system.
Cochlear implant systems have typically consisted of two essential components, an external component commonly referred to as a processor unit and an internal implanted component commonly referred to as a stimulator/receiver unit. Traditionally, both of these components have cooperated together to provide the sound sensation to a user.
The external component has traditionally consisted of a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds into a coded signal, a power source such as a battery, and an external transmitter coil.
The coded signal output by the speech processor is transmitted transcutaneously to the implanted stimulator/receiver unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter coil, which is positioned, to communicate with an implanted receiver coil provided with the stimulator/receiver unit. This communication serves two essential purposes, firstly to transcutaneously transmit the coded sound signal and secondly to provide power to the implanted stimulator/receiver unit. Conventionally, this link has been in the form of a radio frequency (RF) link, but other such links have been proposed and implemented with varying degrees of success.
The implanted stimulator/receiver unit traditionally includes a receiver coil that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlea electrode which applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound. As such, the implanted stimulator/receiver device has been a relatively passive unit that has relied on the reception of both power and data from the external unit to perform its required function.
Traditionally, the external componentry has been carried on the body of the user, such as in a pocket of the user's clothing, a belt pouch or in a harness, while the microphone has been mounted on a clip mounted behind the ear or on the lapel of the user.
More recently, due in the main to improvements in technology, the physical dimensions of the speech processor have been able to be reduced allowing for the external componentry to be housed in a small unit capable of being worn behind the ear of the user. This unit allows the microphone, power unit and the speech processor to be housed in a single unit capable of being discretely worn behind the ear, with the external transmitter coil still positioned on the side of the user's head to allow for the transmission of the coded sound signal from the speech processor and power to the implanted stimulator unit.
The introduction of an external unit able to be positioned behind-the-ear (BTE) provides the user with increased freedom not previously experienced with the more conventional body worn external processor. A BTE unit does not require long cables connecting all of the components together and does not require a separate battery pack, but provides a single unit capable of being discretely worn behind the ear of a cochlear implant user which offers the same functionality of the body worn devices without the obvious restrictions that such devices place upon the user. Due to the obvious benefits such a device offers to the user, it is important that with increasing use of such a component that the reliability of the device be at least the equivalent of the previous body-worn devices. This is especially important with regard to the power supply incorporated in such a BTE device, as whilst it is much smaller, the power supply needs to be sufficient to ensure that the power demands of the implant are met, at least for an acceptable period of time.
As described above, battery cells are typically housed in the external componentry and provide the necessary power for the components of the implant.
In conventional body worn devices, other than BTE devices, the issue of ensuring that the power supply is sufficient to meet the needs of the implant is not of particular concern. This is due in the main to the fact that the size of the component is such that it can accommodate a substantial number of cells and a battery pack can be further employed with such a component. As this component is carried on the body in a harness or the like, the size of the component is not of great importance.
However, with the introduction and increased usage of BTE devices and the desire to provide such devices that are small enough to fit behind the ear of the user or to be discretely worn on the head of the user, the space requirements of the device lead to restrictions in the type and dimension of power supply that can be utilised. Where previously the number of cells required to form the power supply of the device has been relatively unrestricted, such more discrete and compact BTE devices have limited space to house the cells to be used to supply the power for the implant. Where a single battery cell provides insufficient power for all of the components, it has been known to mount two batteries in series within the external componentry.
One type of known battery cell used in cochlear implants and in implants utilising BTE units in particular such as those provided by the present applicant, is the zinc air cell. Such cells have several practical advantages. They have a very high energy density and can supply a device's requirements for a relatively long period of time relative to their size and weight. They also have a relatively constant power output throughout most of their life, thereby reducing the risk of dangerous rapid discharge, such as shorting. Therefore, such cells have particular application to cochlear implants, which utilise these particular advantages. As supplied, these cells do, however, occasionally suffer from a relatively high failure rate. Testing undertaken by the present applicant suggests that approximately 8% of supplied zinc air cells do not perform satisfactorily on delivery. Given that some cochlear implants rely on two satisfactory cells being used in series, the chance that a user will have a problem after replacing a pair of such battery cells increases to 15%. Such problems include finding that the cochlear implant still does not work or stops working satisfactorily after a relatively short time following the insertion of new cells. These problems can then result in the user incorrectly believing that their device has failed and sending the device for repair or replacement, or finding themselves unexpectedly losing their ability to experience hearing sensation after they were sure that the power supply would last for a specific period of time.
When considering the amount of power that the external unit needs to supply to the implant, it should be appreciated that this can vary quite considerably from user to user. The amount of power required by the implant depends on a number of factors. The stimulation rate and speech processing strategy employed by the user dictates greatly the power requirements of the implant. If the implant needs to stimulate at high rates then more power will be required, as will also be the case if a complicated speech processing strategy is to be employed. Further to this, the power requirements are strongly influenced by the thickness of the skin separating the external and internal coils in the transcutaneous link. If this skin flap thickness is large, then the implant will require more power to transmit across such a medium than would be the case if the skin flap thickness is quite small.
In any regard it is important that the external unit is designed such that there is sufficient power available for a wide range of requirements, from those users with large skin flap thicknesses and high rate stimulation strategies to those with small skin flap thicknesses and lower rate strategies. This ensures that an off-the-shelf device can be supplied for all cases without the need for custom-made designs specific to the particular user's power requirements.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.